1. Field of the Invention
The present invention relates to a focus adjustment apparatus that performs focus adjustment using image signals obtained by subjecting an object image formed via an imaging optical system to photoelectric conversion by an imaging element, and an imaging apparatus including the focus adjustment apparatus.
2. Description of the Related Art
Conventionally, as a method for focusing on an object by moving a focus lens, electronic still cameras and the like employ autofocusing (AF) with which focusing operation is automatically performed based on image signals obtained from an imaging element such as a CCD or CMOS. With the AF, if a focus evaluation value is generated using signals from all of pixels obtained from the imaging element, it takes much time for readout. Therefore, there is known a technique that reduces readout time to achieve high-speed AF using image signals obtained by adding up pixel signals of every predetermined number of pixels in a predetermined direction of an image region for thinning out the pixel signals at intervals of the predetermined number of pixels (hereinafter, “thinned-out addition signals”). However, the addition and thinning-out of the pixel signals affect the frequency characteristics of the image signals, resulting in a difference in peak position between the focus evaluation value calculated from image signals obtained from all of the pixels (hereinafter, “non-adding signals”) and the focus evaluation value calculated from the thinned-out addition signals as illustrated in FIG. 12. Consequently, in a case where an image is formed from the signals of all pixels, it is impossible to bring the image into focus even if AF is performed at high speed using the focus evaluation value calculated using the thinned-out addition signals.
There is also known a technique with which two imaging elements are provided, image data is alternately output from the two imaging elements, one of the data is used for control to take a moving image and the other data is used for AF control, and speed enhancement is achieved in the AF control by exposure control suitable for AF as well as pixel addition (Japanese Patent Application Laid-Open No. 2007-097033).
The method in Japanese Patent Application Laid-Open No. 2007-097033 requires the two imaging elements, resulting in an increase in apparatus size and cost. Moreover, in the method in which image data is alternately output from the two imaging elements, both sets of image data cannot be used simultaneously. Furthermore, Patent Application Laid-Open No. 2007-097033 includes no consideration of defocusing that occurs when AF control is performed using the signals resulting from the aforementioned pixel addition.
Therefore, an object of the present invention is to provide a focus adjustment apparatus that can perform high-speed AF with AF accuracy maintained, by performing AF control using a focus evaluation value generated from non-adding signals and a focus evaluation value generated from thinned-out addition signals, which are simultaneously output from one imaging element.